Hydrogen absorption alloy electrode

ABSTRACT

A negative electrode of a battery, chiefly includes hydrogen absorption alloy particles each having a surface layer. The alloy particles satisfy R 2 /R 1 ≧0.004 and 5 μm≦R 1 ≦20 μm, or preferably 5 μm≦R 1 ≦12.5 μm, where R 1  is a half of a median diameter of the particles and R 2  is thickness of the surface layers.

The present disclosure relates to subject matter contained in priorityJapanese Patent Application No. 2001-147596, filed on May 17, 2001, thecontents of which is herein expressly incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydrogen absorption alloy electrodeto be used as a negative electrode of a nickel-metal hydriderechargeable battery.

2. Description of Related Art

Hydrogen absorption alloys, capable of absorbing and releasing hydrogenat and near normal temperatures and pressures, are used as the materialsof negative electrodes in nickel-metal hydride rechargeable batteries.Nickel-metal hydride rechargeable batteries are high in energy densityas compared to nickel-cadmium rechargeable batteries and lead batteries,and have received attention for their cleanness because of containing notoxic element.

Hydrogen absorption alloy electrodes are made of hydrogen absorptionalloy particles, which are produced by crushing alloy ingots coarsely,followed by mechanical grinding in an attritor or the like so that theparticles have a predetermined median diameter of, e.g., 50 μm or so.

The hydrogen absorption alloys form oxide layers easily upon exposure toair. These oxide layers inhibit the absorption and release of hydrogen,thereby yielding the problem that high rate charge-dischargecharacteristics cannot be obtained in initial charge-discharge cycles.For improved alloy activity, there have been disclosed techniques offorming metal layers of nickel on the surfaces of the alloy particles asa hydrogen dissociation catalyst layer. An example thereof appears inJapanese Patent Laid-Open Publication No. Hei 4-137361. Specifically,the technique includes a method of treating hydrogen absorption alloysin a hot alkali solution.

Although given the nickel metal layers on their surfaces as mentionedabove, the conventional hydrogen absorption alloys are greater inaverage particle size and relatively smaller in the surface layerthickness with respect to particle diameters. This means smaller Nicontent on the surfaces of the alloy particles, smaller specific surfacearea per gram of alloy, and smaller surface area per electrode area of 1cm²×thickness of 1 mm. Besides, the surface layers of the alloyparticles contain less metal Ni, or are smaller in the content of metalNi per electrode area of 1 cm²×thickness of 1 mm. This causes problemsof lower activity, higher internal resistance, and poor initial batterycharacteristics in initial cycles.

Under the circumstances, the internal resistance has been lowered tooperable values by repeating low-current charge and discharge aplurality of times on shipment for the sake of initial activation. Asshown in FIG. 1, this repetition of charge-discharge cycles graduallylowers the internal resistance. Nevertheless, it takes long for apredetermined internal resistance to be reached, causing a problem ofdeteriorated productivity.

SUMMARY OF THE INVENTION

In light of the conventional problems mentioned above, an object of thepresent invention is to provide a hydrogen absorption alloy electrodewhich has high activity in initial, charge-discharge cycles.

A hydrogen absorption alloy electrode according to the present inventionchiefly includes hydrogen absorption alloy particles each having asurface layer which mainly includes Ni metal. The alloy particlessatisfy R2/R1≧0.004 and 5 μm≦R1≦20 μm, where R1 is a half of a mediandiameter of the particles and R2 is thickness of the surface layers. Theyet preferable range of R1 is 5 μm≦R1≦12.5 μm.

Consequently, the alloy particles are confined to a range of smallerparticle sizes with an increase in the total surface area. Besides, thesurface layers, which contain large amounts of Ni metal, are given agreater relative thickness. The result is that the surface content of Nimetal increases sufficiently. Thus, as shown in FIG. 1, the initialactivation time necessary to lower the internal resistance to operablevalues is made shorter than heretofore, with an improvement inproductivity.

Here, R1 of alloy particles 1, which have various shapes as shown inFIG. 2A, refers to the median of radii of alloy particles 1 a. The alloyparticles 1 a are spheres having the same volumes or circles having thesame cross-sectional areas which are assumed from the respective alloyparticles 1. Surface layers 3 are ones different from bulk layers 2 incomposition or texture, being formed by immersing alloy particles in ahot alkali aqueous solution so that misch metals, Co, Al, and Mndissolve from the surfaces of the hydrogen absorption alloys. Thesurface layers 3 contain Ni metal, along with misch metal hydroxides andoxides.

The internal resistance is further lowered to shorten the initialactivation time by rendering the alloy particles greater than or equalto 0.5 m²/g in specific surface area, the surface area greater than orequal to 0.28 m² per electrode area of 1 cm²×thickness of 1 mm, thesurface layers of the alloy particles greater than or equal to 1.5% byweight in the content of metal Ni, and the surface layers of the alloyparticles greater than or equal to 8×10⁻³ g in the content of metal Niper electrode area of 1 cm²×thickness of 1 mm.

While novel features of the invention are set forth in the preceding,the invention, both as to organization and content, can be furtherunderstood and appreciated, along with other objects and featuresthereof, from the following detailed description and examples when takenin conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing variations in internal resistance of ahydrogen absorption alloy electrode of the present invention and of aconventional example due to charge-discharge cycles;

FIGS. 2A and 2B are explanatory diagrams showing the actual shape of ahydrogen absorption alloy particle, a median diameter, and the thicknessof a surface layer;

FIG. 3 is a graph showing the correlation between R2/R1 and the internalresistance of hydrogen absorption alloy electrodes according to anembodiment of the invention;

FIG. 4 is a graph showing the correlation among R2/R1, the specificsurface area, and the internal resistance of the hydrogen absorptionalloy electrodes according to the embodiment;

FIG. 5 is a graph showing the correlation between the per-thicknesssurface area and the internal resistance of the hydrogen absorptionalloy electrodes according to the embodiment;

FIG. 6 is a graph showing the correlation among R2/R1, the surfacecontent of metal Ni, and the internal resistance of the hydrogenabsorption alloy electrodes according to the embodiment; and

FIG. 7 is a graph showing the correlation between the surface content ofmetal Ni per thickness and the internal resistance of the hydrogenabsorption alloy electrodes according to the embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, an embodiment of the hydrogen absorption alloy electrode ofthe present invention will be described with reference to FIGS. 3 to 7.

The hydrogen absorption alloy electrode of the present invention useshydrogen absorption alloys which are of no limited composition inparticular. In the present embodiment, hydrogen absorption alloy powdersof misch metal Ni_(3.5)Co_(0.7)Mn_(0.4)Al_(0.3), containing 45% byweight of Ce, 30% by weight of La, 5% by weight of Nd, and 20% by weightof other rare-earth elements, were used.

These hydrogen absorption alloy powders were prepared in the followingway. Initially, the misch metal and other metal materials were put intoan arc melting furnace in ratios corresponding to the foregoing alloycomposition. Under a reduced pressure of 0.0133 to 0.00133 Pa (10⁻⁴ to10⁻⁵ Torr), the materials were heated to melt in an argon gas atmosphereby means of arc discharge. The resultant was subjected to further heattreatment in the argon gas atmosphere at 1050° C. for eight hours, andcooled into an alloy. Next, this alloy was coarsely crushed and thenground in a ball mill to or below various particle sizes, so thathydrogen absorption alloy powders of 10, 25, 32, and 40 μm in mediandiameter (2×R1) were made. The method of measuring a median diameter ofthe alloy powders is not limited to particular one. For example, laserdiffraction scattering method may be employed.

Subsequently, these hydrogen absorption alloy powders were immersed in a90° C.-heated KOH aqueous solution of 1.3 in specific gravity fordifferent periods (30, 60, and 90 minutes) before rinsed with water anddried for surface modification (the formation of the surface layers 3).A thickness of the surface layers can be measured by transmissionelectron microcopy, although the method of measuring is not limited toparticular one.

With respect to 100 parts by weight of each hydrogen absorption alloypowder modified, 0.15 parts by weight of carboxymethyl cellulose, 0.3parts by weight of carbon black, and 0.7 parts by weight ofstyrene-butadiene copolymer were added. The resultants were kneaded withwater into pastes.

These pastes were applied to punched metals in thicknesses of 260, 300,and 400 μm. After dried, the articles were pressed in a roll press andthen cut into negative electrodes of predetermined size.

These negative electrodes were combined with positive electrodes andseparators to fabricate 6.5-Ah prismatic cells. Here, the positiveelectrodes were foamed nickel plates filled with an active materialchiefly including nickel hydroxide. The separators were made ofpolypropylene nonwoven fabric sulfonated.

Samples A-L fabricated as described above were measured for physicaldata including R1 (μm), R2 (μm), surface area (m²/g), R2/R1, and thesurface content of metal Ni (wt %), and the internal resistance (mΩ)after 10 cycles of initial activation. Table 1 shows the measurements.

TABLE 1 Sample A B C D E F G H I J K L Median 10 10 10 25 25 25 32 32 3240 40 40 diameter (μm) R1 (μm) 5 5 5 12.5 12.5 12.5 16 16 16 20 20 20Immersion 30 60 90 30 60 90 30 60 90 30 60 90 period (min.) R2 (μm) 0.020.04 0.05 0.02 0.05 0.07 0.02 0.06 0.08 0.02 0.07 0.08 Surface 1.25 1.301.33 0.50 0.52 0.53 0.41 0.39 0.41 0.33 0.33 0.33 area (m²/g) R2/R10.004 0.006 0.010 0.002 0.004 0.005 0.002 0.004 0.005 0.001 0.004 0.004Surface 3.0 3.4 3.7 1.2 1.3 1.5 0.9 1.0 1.2 0.8 0.8 0.9 content of metalNi (wt %) Internal 5.0 4.2 3.9 6.8 5.2 5.1 7.5 5.5 5.3 7.7 5.5 5.5resist- ance (mΩ)

For the sample F, hydrogen absorption alloy pastes applied inthicknesses of 260, 300, and 400 μm were pressed into samples F-1 to F-6of different thicknesses. These samples F-1 to F-6 were measured for thespecific surface area (m²/mm) per 1-mm-thick alloy portion in a 1-cm²area of negative electrode, the content of metal Ni (g/mm) per1-mm-thick alloy portion in a 1-cm² area of negative electrode, and theinternal resistance (mΩ). Table 2 shows the measurements, along with thespecific surface area (m²/g) and the content of metal Ni (wt %) of thealloy shown in Table 1.

TABLE 2 Sample F-1 F-2 F-3 F-4 F-5 F-6 Application 260 260 300 300 400400 thickness (μm) Post-press 211 190 244 219 320 288 thickness (μm)Specific surface 0.53 0.53 0.53 0.53 0.53 0.53 area of alloy (m²/g)Specific surface 0.39 0.44 0.34 0.38 0.26 0.28 area of negativeelectrode (m²/mm)* Content of metal 1.5 1.5 1.5 1.5 1.5 1.5 Ni in alloy(wt %) Content of metal 0.0110 0.0122 0.0095 0.0106 0.0073 0.0081 Ni innegative electrode(g/mm)* Internal 4.4 4.0 5.1 4.1 7.5 5.4 resistance(mΩ) Remarks Table1-F *per thickness of 1 mm (1 mm × 1 cm × 1 cm)

Now, analyses will be given with reference to FIGS. 3 to 7, in which theforegoing measurements are plotted. FIG. 3 shows the correlation betweenR2/R1 and the internal resistance. The smaller R2/R1 is, the higher theinternal resistance becomes. The greater R2/R1, the lower the internalresistance. At R2/R1 of 0.004 and greater, the internal resistance iscontrolled to 5.5 mΩ or below. This allows a reduction in the initialactivation time, thereby improving productivity. Note that when R1reaches or exceeds 20 μm, R2/R1≧0.004 is unattainable even by extendedperiods of surface modification.

FIG. 4 shows the case where the specific surface areas of the alloys aretaken into account as an additional parameter. Where R2/R1≧0.004 and thespecific surface area reaches or exceeds 0.5 m²/g, the internalresistance is stably reduced to or below 5.5 mΩ.

FIG. 5 shows the correlation between the specific surface area (m²/mm)per 1-mm-thick alloy portion in a 1-cm² area of negative electrode andthe internal resistance. The internal resistance increases sharply whenthe amount of pressing is smaller and the specific surface area perthickness of 1 mm falls below 0.28 m²/mm. At 0.28 m²/mm and above, theinternal resistance is controlled to 5.5 mΩ or below.

FIG. 6 shows the correlation to the internal resistance with R2/R1 andthe surface content of metal Ni in the alloy as parameters. WhereR2/R1≧0.004 and the surface content of metal Ni reaches or exceeds 1.5%by weight, the internal resistance is stably reduced to or below 5.5 mΩ.

FIG. 7 shows the correlation between the surface content of metal Ni(g/mm) per 1-mm-thick alloy portion in a 1-cm² area of negativeelectrode and the internal resistance. The internal resistance increasessharply when the amount of pressing is smaller and the surface contentof metal Ni per thickness of 1 mm falls below 0.008 g/mm. At and above0.008 g/mm, the internal resistance is controlled to 5.5 mΩ or below.

The misch metal preferably is an intermetallic compound having astoichiometric ratio generally represented as LaNi₅, wherein part of Lais replaced with Ce, Pr, Nd, and/or other rare-earth elements and partof Ni is replaced with such metals as Co, Mn, and Al.

The method of manufacturing the hydrogen absorption alloy ingots is notlimited to particular one. In view of low manufacturing costs, themanufacturing method of melting and casting metals into molds ispreferable. Other methods such as quenching can also be used, however,even with equivalent or greater effect.

The alloy obtained may be mechanically wet-ground in water or in anaqueous solution by using an attritor or the like. The wet grindingsuppresses local oxidation on the alloy surfaces than with the dry. Thisfacilitates conducting subsequent treatments more uniformly.

Aside from KOH, the surface modification may use such alkali metalhydroxides as NaOH, with a preferable range of concentrations of 10-60%by weight. During the treatments, the solution temperature preferablyfalls within the range of 60-140° C., and the immersion period 0.5-5hours.

According to the hydrogen absorption alloy electrode of the presentinvention, the alloy particles are confined to smaller particle sizeswith an increase in the total surface area. Besides, the surface layers,containing large amounts of Ni metal, are given a greater relativethickness. The surface content of Ni metal thus increases sufficiently.This allows a reduction in the initial activation time necessary tolower the internal resistance to operable values, thereby improvingproductivity.

Although the present invention has been fully described in connectionwith the preferred embodiment thereof, it is to be noted that variouschanges and modifications apparent to those skilled in the art are to beunderstood as included within the scope of the present invention asdefined by the appended claims unless they depart therefrom.

What is claimed is:
 1. A hydrogen absorption alloy electrode comprising:hydrogen absorption alloy particles each having a surface layer, whereinsaid alloy particles satisfy R2/R1≧0.004 and 5 μm≦R1≦20 μm, where R1 isa half of a median diameter of said alloy particles and R2 is thicknessof said surface layers, wherein said alloy particles are greater than orequal to 0.5 m²/g in specific surface area.
 2. A hydrogen absorptionalloy electrode, reaching or exceeding 0.28 m² in the surface area perelectrode area of 1 cm²×thickness of 1 mm, comprising: hydrogenabsorption alloy particles each having a surface layer, wherein saidalloy particles satisfy R2/R1≧0.004 and 5 μm≦R1≦20 μm, where R1 is ahalf of a median diameter of said alloy particles and R2 is thickness ofsaid surface layers.
 3. A hydrogen absorption alloy electrodecomprising: hydrogen absorption alloy particles each having a surfacelayer, wherein said alloy particles satisfy R2/R1≧0.004 and 5 μm≦R1≦20μm, where R1 is a half of a median diameter of said alloy particles andR2 is thickness of said surface layers, wherein said surface layers ofsaid alloy particles are greater than or equal to 8×10⁻³ g in thecontent of metal Ni per electrode area of 1 cm²×thickness of 1 mm.